Electrodeposition of zinc



United States Patent 8 2,975,112 I ELECTRODEPOSITION oF' ZINC Edward B. Saubestre, Elmhurst, N.Y., assignor, by nfesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware No Drawing. Filed Jan. 4, 1955, Ser. N0.,419,842

4 Claims. (Cl. 204119) This invention relates to the electrodeposition of zinc, and more particularly to the electrowinning of zinc from zinc solutions containing germanium as an impurity.

Gemanium is a metal which at present is in great demand because of its extremely valuableproperties in electrical and electronic circuits. Accordingly, ores containing this metal are being currently processed in great quantities. Such ores usually contain relatively large amounts of zinc, whereby zinc sulfate residues in solution form are obtained. Similar solutions are obtained as an intermediate step in the extraction of other metals, including zinc itself, from ores or other materials with sulfuric acid.

The art of recovering zinc from acid solutions of zinc sulfate by electrolytic means is of course old and well known. However, in attempting to extract zinc by such means from zinc sulfate solutions (obtained as a residue from the manufacture of germanium), difficulty was .encountered. Little or no plating of zinc on the'cathode could be achieved. This difficulty was traced to the presence of small quantities of germanium present in the zinc sulfate solution, despite all the previous steps taken to extract the germanium. Amounts of germanium as low as 1 mg./l. were found to be suflicient to prevent deposition of zinc. 1

While the presence of certain impurities in electrolytesbasic salts or other impurities, Accordingly, mostfexpedie'nts heretofore employed for controlling pH have some electrolytic processes. deliberately, joperateat anv is known often to cause great difficulty duringthe operation of plating by electrolytic methods, most if not all of the expedients heretofore employed for solving this problem have been directed towards a preliminary treatment of the electrotyte to remove the undesired impurities. As examples of some of these proposed preliminary purification methods, reference may, be had to US. Patents Nos. 2,396,569, 2,405,302, 2,503,479, 2,509,916, 2,509,917, and 2,509,918. Although these methods have been generally found to be ineffective for removing impurity. Other objects and advantages will appear as the description proceeds.

The attainment of the above objects ismade' possible by the instant invention which comprisesmaintaining'the pH of the electrolyte within a range of about 3.0 to 3.5 during electrowinning. While this may be accomplished in any convenient way, it has been found that best resuits may be obtained by adding a strong inorganicbase' to'the electrolyte in such manner as to maintain the pH within a range of about 3.0 to 3.5 during the electrolysis.

This method has been found to enable satisfactoryplating of zinc over extended periods of time despite. the] presence of up to about 0.01 g./l., or more of germanium oxide in the electrolyte. When the pH is allowed to go' below about 3.0, the cathode elficiency begins ,to drop rapidly, while at a pH of about 4 or more, a heavy pre'-' cipitate forms with consequent drop in efliciency.

The improved results attained by the instantjprocess are surprising in view of the many advantages attributable to operation at high acid concentration (low PH). Thus,

a high acid concentration increases the conductivity of involved use of acids of various types, acidsalts', soluble anodes, diaphragms, and the like. As a matter of fact,-

extremely low pH depending upon the continuous formation of free acid known-to occur when usinginsoluble anodes, for use in a recyclingprocess to dissolve further quatities of zinc from ores containing same.

In carrying out a preferred embodiment of of the instant invention, a strong inorganic base such as lithium hyroxide but more preferably sodium or potassium hydroxide, is added periodically or continu ous ly as needed to maintain the pH of the bath within the required range.-

trated-a'queous solution to promote more rapid-neutraliza-' tion and pH control without unduly diluting the bath. For most practical purposes, concentrations of about 15 to by'w'eight are operative, a rangeof ahdut germanium from the zinc electrolyte solutions being w dealt with, one such ure-purification method has been found satisfactorily employed to remove the germanium. It involves absor ing and removing the germanium from solutidn by means of ferric hydrate precipitated by zinc oxide in the leaching operation.

The aforementioned pro-purification method for removing the undesirable germanium has, however, several disadvantages. In common with other pre-purification methods, it involves additional equipment and steps, in-

cluding a filtration which is not easily effected. Further, it requires the deliberate addition of iron to ores which may have a relatively high germanium content and a low iron content which also introduces another element to control with the attendant problem of human error in actual plant operation.

20 to 40% being preferred.

The pH or the bath should beiperiodically or continu ou'sly measured, and this may be carried out. in any manner well known in the art.

employed to determine when the pH approaches or falls below this value.

Change of color of the indicator as the pH of the elecor by automatically controlled means.

then be made manually at periodical intervals, or either periodically or continuously by automatic means-and the pH measured only occasionally as a check.

Automatic control'is of course highly desirable and should be made use of where possible. .Merely by way Patented Ma r. 1.4, 1 961 the process Although the base may be addedasa'. solid, it is preferably added in the form of a'concefi Since insoluble anodes are o I employed, the tendency of the pH is to fall belowjthe minimum value of about 3.0 as required herein; and chemical colorimetric indicators or test papers may be As such indicators, there may be mene tioned Butter Yellow, Congo Red, 2.5-dintrohydroquinone, Bromophenol Blue, Methyl Orange and the like.

of illustration, means may be installed for automatically extracting a test portion of the electrolyte solution, and measuring its pH by proper instrumentation. Means responsive to the measured pH may then be provided for automatically adding the base to the electrolyte solution in predetermined increments sufficient to prevent the pH from falling below the minimum value of about 3.0.

The electrolytic process is otherwise carried out in a well known manner. Insoluble anodes are employed, such as lead, lead-silver, silicon-iron, lead-thallium-calcium, lead peroxide, manganese dioxide, or the like. Choice of material for the cathode will of course depend upon the object in view, which may be to recover zinc in substantially pure form, to plate articles with zinc, or the like. In the latter case, the article to be coated, which may be iron or steel, aluminum, zinc or the like, is employed as the cathode. In the former case, the cathode should be of such nature as to permit the zinc to be readily detached therefrom, as for example, oiled or graphite-covered copper or aluminum. Alternately, the cathode may be electrolytic zinc foil, whereby it may be melted down as a unit. Another expedient which may be employed involves the use of iron or other high melting point material as the cathode followed by removal of the zinc deposited thereon by dipping the cathode in a bath of molten zinc, fused zinc chloride or the like. Current densities of the order of about 50 to 600 amperes per square foot may be used with good results. The zinc content of the bath is replenished from time to time by addition of more zinc sulfate solution which may contain germanium as an impurity as abovementioned. The pH of the replenishing zinc sulfate solution may vary from the range required in the instant invention provided of course it is not added in such a manner as to carry the pH of the electrolytic bath outside said range.

The following example is illustrative of a preferred embodiment of this invention and is not to be regarded as limitative:

Example A zinc sulfate solution, obtained as a residue from the manufacture of germanium at Towanda, Pa. was examined and found to have a pH of 4.0 to 4.5 and to contain 100 g./l. of zinc and .006 g./l. of GeO This solution was employed as the electrolyte bath for the electrodeposition of zinc using platinum anode and 1010 cold rolled steel cathode at a current density of 80 amperes per square foot, and at a pH maintained within the range of 3.1 to 3.3 (colorimetric) by measurement with indicator paper (2.74.7) made by Anachemia, Ltd. and periodic addition of a 24% by weight aqueous solution of sodium hydroxide. The process was run for several days, with replenishment of metal content from time to time by addition of further amounts of the original zinc sulfate solution. Good quality plates of zinc in thicknesses of up to 1.5 mils were obtained. at cathode efiiciencies of 80% to 90%, with substantially no loss in efliciency or change in appearance of plate throughout the run.

This invention has been disclosed with respect to certain preferred embodiments, and various modifications and variations thereof will become obvious to the person skilled in the art. It is to be understood that such modifications and variations are to be included within the spirit and purview of this application and the scope of the appended claims.

What I claim is:

l. A method of extracting zinc from baths containing zinc salts and germanium as an impurity including the steps of making said bath containing zinc salts and germanium the electrolyte in the presence of an insoluble anode and a cathode, raising the pH of said bath to within the range of 3.0 to 3.5 and thereafter maintaining the pH within said range, and electrowinning zinc at said cathode despite the germanium content of said bath.

2. A method of extracting zinc from baths containing zinc salts and germanium as an impurity including the steps of making said bath containing zinc salts and germanium the electrolyte in the presence of an insoluble anode and a cathode, raising the pH of said bath to within the range of 3.0 to 3.5 by the addition of a strong inorganic base and thereafter maintaining the pH within said range, and electrowinning zinc at said cathode despite the germanium content of said bath.

3. A method of extracting zinc from baths containing zinc salts and germanium as an impurity including the steps of making said bath containing zinc salts and germanium the electrolyte in the presence of an insoluble anode and a cathode, raising the pH of said bath to within the range of 3.0 to 3.5 by the addition of alkalimetal hydroxide selected from the group consisting of sodium and potassium hydroxide and thereafter maintaining the pH within said range, and electrowinning zinc at Zaid cathode despite the germanium content of said 4. A method of extracting zinc from baths containing zinc salts and germanium as an impurity including the steps of making said bath containing zinc salts and germanium the electrolyte in the presence of an insoluble anode and a cathode, raising the pH of said bath to within the range of 3.0 to 3.5 by the addition of a concentrated aqueous solution containing an alkali-metal hydroxide and thereafter maintaining the pH within said range, and electrowinning zinc at said cathode despite the germanium content of said bath.

References Cited in the file of this patent UNITED STATES PATENTS 2,576,998 Main et al. Dec. 4, 1951 FOREIGN PATENTS 693,606 Great Britain July 1, 1953 

1. A METHOD OF EXTRACTING ZINC FROM BATHS CONTAINING ZINC SALTS AND GERMANIUM AS AN IMPURITY INCLUDING THE STEPS OF MAKING SAID BATH CONTAINING ZINC SALTS AND GERMANIUM THE ELECTROLYTE IN THE PRESENCE OF AN INSOLUBLE ANODE AND A CATHODE, RAISING THE PH OF SAID BATH TO WITHIN THE RANGE OF 3.0 3.5 AND THEREAFTER MAINTAINING THE PH WITHIN SAID RANGE, AND ELECTROWINNING ZINC AT SAID CATHODE DESPITE THE GERMANIUM CONTENT OF SAID BATH. 